Feed on Fire. If the elemental takes fire damage from a spell or other magical effect, its size increases by one category. If there isn’t enough room for the elemental to increase in size, it attains the maximum size possible in the space available. While the elemental is Large or bigger, it makes Strength checks and saving throws with advantage.

If the elemental starts its turn at Gargantuan size, the elemental releases energy in an explosion. Each creature within 30 feet of the elemental must make a DC 13 Dexterity saving throw, taking 28 (8d6) fire damage on a failed save, or half as much damage on a successful one. The explosion ignites flammable objects in the area that aren’t being worn or carried. The element's size then becomes Medium.

Form of Fire and Water. The elemental can move through a space as narrow as 1 inch wide without squeezing. In addition, the elemental can enter a hostile creature’s space and stop there. The first time the elemental enters another creature’s space on a turn, that creature takes 5 (1d10) fire damage and catches fire; until someone takes an action to douse the fire, the burning creature takes 5 (1d10) fire damage at the start of each of its turns.

Heated Body. A creature that touches the elemental or hits it with a melee attack while within 5 feet of it takes 5 (1d10) fire damage.

Multiattack. The elemental makes two slam attacks.

Fire Slam. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) bludgeoning damage plus 7 (2d6) fire damage.

Ice Slam. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) bludgeoning damage plus 7 (2d6) cold damage.

Heavy Slam attacks can only be used if the elemental is Huge or larger.

Heavy Fire Slam.  Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 11 (2d8 + 3) bludgeoning damage plus 14 (4d6) fire damage.

Heavy Ice Slam. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 11 (2d8 + 3) bludgeoning damage plus 14(4d6) Ice damage.

Spell Vitalization. Immediately after a creature casts a spell of 1st level or higher within 120 feet of the creation, the elemental can move up to twice its speed without provoking opportunity attacks. It can then make one slam attack against a target of its choice.